The aim of this study is to investigate environmental, biologic, and genetic determinants of bone mass in a large (n=650) biracial of young women enrolled in the NHLBI Growth and Health Study. Eleven-year longitudinal data are available data are available on dietary intake, physical activity, body composition, pubertal milestones, pregnancy, oral contraceptive use, smoking, and alcohol intake. This cohort is also concurrently enrolled in a second study which obtains annual total body DEXA scans until year 2000. Nested within this study, we will 1) determine what environmental (current and antecedent) and genetic factors determine bone mass (BMD) at the lumbar spine and proximal femur at ages 20-22 years; 2) determine racial difference in the effects of the significant environmental and genetic factors found in aim #1; and 3) determine what environmental and genetic factors influence biochemical markers of bone metabolism and calcium homeostasis. Bone mass will be assessed several different ways in part to address the biethnic nature of the population. Genotyping will be done for variation at genetic loci intimately involved in bone metabolism and calcium homeostasis. Biomarkers include serum osteocalcin and ionized calcium, intact parathyroid hormone, and urinary N-telopeptides and calcium. The main hypotheses involve the effects of dietary calcium, vitamin D, physical activity, and pubertal maturation on BMD, and their interaction with each other as well as with genotypes. Attention will be directed to understanding the extent to which selected environmental and genetic factors influence the well-recognized black-white differences in BMD. The relative importance of the timing of "exposure" to these environmental factors will be assessed by analyzing data in biologically meaningful time periods defined by pubertal milestones. This study will provide a comprehensive evaluation of the role of an exhaustive set of environmental measures, genotypes, and biomarkers of bone/calcium homeostasis in determining BMD as well as in determining the racial differences in BMD. Thus, by capitalizing on the efficiency and cost- saving afforded by the two ongoing studies at Cincinnati Children's Hospital, our study can "telescope" the 11 years of follow-up to uncover important childhood antecedents of peak bone mass in black and white women.